The present invention relates to the field of hydrogen generation by partial oxidation of hydrocarbon mixtures (a technology usually referred to as "POX" in the literature, standing for Partial Oxidation). The invention relates more particularly to the processes and plants for the simultaneous generation of hydrogen and energy.
Many processes have been reported in the literature, relating to the production of hydrogen and/or energy, on the basis of partial oxidation of hydrocarbon mixtures (such as coal, natural gas, naphtha and heavy fuel oil), and giving rise, depending on the reaction conditions (temperature, pressure and composition of the reactive gases), to a highly variable composition of the reaction gas produced. Reference will be made to the many articles published in the name of TEXACO, SHELL or WESTINGHOUSE.
The reaction gas produced as output from a partial oxidation unit usually contains several tens per cent of the H.sub.2 /CO mixture, but also nitrogen, CO.sub.2, argon, etc.
The available processes usually include, downstream of the partial oxidation unit, one or more reaction gas purification steps: removal of CO generally by catalytic reaction with steam, of all or part of its H.sub.2 S, of COS (carbonyl sulphide) and of NOx (purification steps well known to those skilled in the partial oxidation art).
Downstream of these purification steps, the reaction gas thus purified is usually sent to a preferential adsorption separator (for example of the PSA type) or else to a membrane-type separator, so as to produce, according to the required specification in each case, energy or a hydrogen stream or a stream of CO.
Among the oxidants most often cited in the literature regarding POX are air or air enriched in oxygen up to 35% or even more.
From this copious literature, mention may be made of the document EP-A-217,505 which describes the simultaneous generation of energy and of a mixture having at least 50% hydrogen, the mixture containing the hydrogen being produced as the "unadsorbed product" stream from a preferential adsorption separator, in which the reaction gas produced as output from a POX unit has been treated, the energy being moreover produced by sending the gas mixture adsorbed in the separator (rich in CO, CO.sub.2, N.sub.2, CH.sub.4, etc) into a unit for the generation of electricity by catalytic combustion followed by expansion in a gas turbine.
Since the objective of this document is to provide a mixture containing hydrogen used for the synthesis of ammonia, relatively low concentrations of argon are tolerated. The author however mentions that the gas generated by the adsorption separator contains non-negligible quantities of argon (which derives from the oxidant gas supply used) , argon that the document declares is tolerated without any inconvenience.
Work successfully carried out by the Applicant in this field has confirmed that, in the case of such argon-containing oxygen supplies, this argon is largely found in the hydrogen-rich mixture produced by the process. However, although in many subsequent applications using such a hydrogen-rich mixture the argon presents no inconvenience, in other cases (such as hydrodesulphurization or hydrocracking processes), the argon lowers the partial pressure of the hydrogen in the gas mixtures used, giving rise to markedly inferior reactions.
Faced with this problem, it may be firstly envisaged to use very pure (typically 99.5%) oxygen, thus further increasing the operating costs of the unit.
Another technically achievable solution might consist in sizing a PSA preferential adsorption unit so as to stop the argon much better, but such sizing would then be not without consequences with regard to the cost of the plant and to its performance (reduction in the hydrogen extraction yield and increase in the quantity of low-value low-pressure adsorbed retentate gas).